This all started with a report from a user about Class::Accessor::Grouped being much slower on 5.10 compared to 5.8.8.

At this point, I call 'not it'...but only because I really don't know what in the hell is going on.
Two brand new fresh FreeBSD 6.2 installs. One with a a compiled (not ports) Perl 5.8.8 and the other with a compiled 5.10.0.

The test case is simple set_inherited, sans C::A::G in the picture, since the reporter noted set was slower as well (which BTW has no MRO interaction):